Not the greatest of mods, but it DID keep the filter from oscillating rail-2-rail. <ouch>

Just remembered! I took it down because other folks were claiming credit.Also took down the EnvelopeFinishingTrick on the 9700s for the same reason. PM me thru here or thru the YahooPAIAGroup with your email and I can send it to you. The Group has lots of interesting PAIA stuff but is not affiliated with PAIA the company.

What is needed is a feedback limiter, but I have not had any time to persue. This is done on almost all the other MM-VCFs that I've seen.Sometimes it's done with an opamp stage and some limiting diodes.Other times it's done with an OTA acting as a limiter/VCA.

tone(man)/toneboy22 aka TonyB

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Sun Sep 08, 2013 12:54 pm

enossified

Joined: Thu Aug 29, 2013 7:35 pmPosts: 7

Re: 9700 filter mod---zener diodes---HELP

Crap, no PM button, FAQ says "contact admin"

Waiting on membership approval at Yahoo group

Mon Sep 09, 2013 4:16 pm

toneman

Joined: Thu Sep 24, 2009 2:14 pmPosts: 90

Re: 9700 filter mod---zener diodes---HELP

Yep, the PAiAGroupAdmin is usually pretty quick.

I'll still be here.

Do you know about the 9730 trigger mod that I did??

T

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Mon Sep 09, 2013 6:02 pm

PAiA-Scott

Joined: Sun Nov 09, 2008 2:49 pmPosts: 732

Re: 9700 filter mod---zener diodes---HELP

It's best if you can just set the controls and modulation amounts up to or below the onset of this runaway/feedback condition rather than changing the circuit.

With your patch through the 9730 set-up run through all anticipated frequencies of the signal source, and, the setting(s) for the amount of envelope/repeating-envelope (and any external modulation via control input connectors) as the Fc and Q settings are advanced on the first, then second (if cascaded) filter sections.

Think of it as a post calibration, "calibration procedure" and with any more input amplitude or difference in frequency or modulation to vary the filter's initial frequency setting, it will enter the critical condition, but reduced settings and amounts should all be within the limits.

Reducing the amplitude of the input signal makes the settings less critical as there will be more headroom for boost at the center or corner frequencies.

, a general, monophonic analog synthesizer patch is described with the two oscillator signals first going through the VCA L and R mixer.

The L+R output is a passive mix and so there is some attenuation and connecting this to the filter input allows for more dynamics through this section.

Even more attenuation of the signal input to the VCF would ensure things didn't go over the edge.

This can be done through an added module or potentiometer, or, by substituting a stereo (trs) connector on the filter input which would work to connect attenuating resistance to ground as a mono (ts) plug is inserted and shunts the ring and sleeve contacts.

A trs plug with no connection to the ring terminal would allow normal, non-attenuated input.

See the schematic in the Quadrafuzz manual for an example of the way a High Level input signal is attenuated via connection to the TRS input connector.

Note the setting of the scale trim helps keep current peaks with variations in control voltages (Fc, Modulation) in check. I think these are a way it can break into oscillation even if the input signal is attenuated.

I didn't investigate any diode limiting since, it wasn't so clear as to where this might be best applied, other than the input and with it mainly working like a fuzz circuit, it would be making all the input waves like square waves unless it got fairly complex.

This 6790 kit could be put behind an FR-blank for use as a module working to prevent too much signal from getting to following devices. It attenuates, rather than clips, by an amount needed to keep the signal below an adjusted Limit Point.

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